Fig. 1.

Schematic of muscle–tendon unit work loop experimental apparatus and protocol. (A) Schematic of experimental MTU work loop preparation. A bullfrog plantaris MTU was attached using an aluminium clamp and steel cable to an ergometer and driven through sinusoidal length change cycles while an electrical stimulus was applied to the sciatic nerve over set time periods during a cycle. Sonomicrometry was used to directly measure the length change of the muscle fascicles (CEs) within the MTU. (B) Schematic showing the imposed MTU length change trajectory (frequency 2 Hz, amplitude 4 mm, positive is lengthening) (green) and the set of conditions for onset timing and duration of muscle stimulation (orange) explored in this study. The onset timing of muscle stimulation (i.e. muscle stimulation phase) was defined with respect to the MTU length change cycle, with 0% referring to MTU minimum length (i.e. ‘bottom dead center’) and 50% referring to MTU maximum length (i.e. ‘top dead center’). We tested muscle stimulation phases spanning from −37.5% to +50% and stimulus always had a 50 ms duration (i.e. 10% duty) (orange).